Axial-flow turbines have a function to convert the kinetic energy which is generated as a high-pressure fluid expands toward a low-pressure area, into a turning force by stages comprised of stationary blades and moving blades. In axial-flow turbines, in order to increase work output per stage, it is desirable to increase the flow rate as the mass of a fluid flowing per unit time. If work output per stage is increased, production of electricity can be increased without altering the number of stages in the case of multi-stage turbines such as steam turbines for power generation.
In order to increase the flow rate, it is useful to increase the annular band area as the area of a fluid flow zone as seen from the rotation axis. For axial-flow turbines, the annular band area is calculated as follows: the average diameter obtained by dividing the sum of blade outer peripheral end diameter and inner peripheral end diameter by 2 is multiplied by blade length and the product is multiplied by the circle ratio. Therefore, in the case of axial-flow turbines, in order to increase the annular band area, the blade length and average diameter are increased.
If the blade length or average diameter is increased, the moving blade tip circumferential speed increases and the relative velocity at fluid inflow to the moving blade becomes supersonic, which may cause shock wave loss in the inflow area of the moving blade.
In the past, a technique to reduce shock wave loss in the moving blade inflow area due to a lengthened turbine moving blade as described in PTL 1 has been proposed in which the shape of the annular outer peripheral portion of the stationary blade is designed so as to prevent the velocity of a fluid flowing to the moving blade relative to the moving blade from exceeding sonic velocity.